By the year 1991, the particulate emission standards set by the Environmental Protection Agency (EPA) will require all urban buses to emit less than 0.1 gm/hp-hr of particulate matter. The same standard will apply to heavy duty trucks in 1994. These particulates are very small in size, with a mass median diameter in the range of 0.1-1.0 micrometers, and are extremely lightweight. Particulate traps have been developed which are effective to remove a sufficient quantity of the particulates from the exhaust gas of a typical diesel engine for a truck or bus to bring the exhaust emissions into compliance with the EPA regulations.
During normal operations of a typical vehicle engine, approximately 20 cubic feet of particulate matter must be trapped per 100,000 miles of vehicle operation. Obviously this particulate matter cannot be stored within the vehicle. Therefore, successful long term operation of a particulate trap-based exhaust aftertreatment system (EAS) requires some method for removal of the trapped particulates. One method which has proven to be successful has been to provide means to burn off the trapped particles to regenerate the filter. The regeneration process is carried out during normal operation of the trap by the delivery of additional heat to the inlet of the particulate trap at a temperature in excess of 1200 degrees Fahrenheit. The process results in oxidation of the filtered carbonaceous particulates in a manner that restores the trap's "clean" flow restriction. However, the regeneration process also unavoidably produces temperature gradients, thermal expansion and resultant thermal stresses in the particulate trap.
The differences in thermal expansion within and between the various components of a particulate trap during both normal operation and regeneration is an important consideration in the design and production of a trap mounting system. A conventional particulate trap mounting assembly includes a trap element such as a porous ceramic material wrapped with a resilient material and compressed in an outer housing shell. Adequate radial pressure or gripping force by the housing on the trap is necessary to withstand vehicle vibration and pressure forces and to prevent axial dislocation of the trap under all expected operating conditions. One particular type of particulate trap mounting system is disclosed in U.S. Pat. No. 4,504,294 issued to Brighton. Brighton discloses a particulate trap having a ceramic trap element covered with a resilient material identified as INTERAM.RTM., and mounted in a "clam shell" housing. This "clam shell" construction uses two complementary half shell portions with flanges which are designed to be clamped and held together to compress the trap in a fixed diameter cavity. Therefore, the amount of radial mounting pressure exerted on a given trap element by this type of clam shell housing will be significantly affected by the trap element outside diameter, the thickness and compliance of the resilient material and the clam shell inside diameter. Each of these dimensions have manufacturing tolerances which must be carefully controlled to insure that adequate, but not excessive, radial pressure is applied to the particulate trap element. Because of the great difficulty associated with holding very close manufacturing tolerances, a significant percentage of trap assemblies are either much too tight causing damaging axial tensile loads on the trap during temperature variations or too loose for reliable axial retention of the trap.
Another common particulate trap assembly is the "stuffed-can" type, also disclosed in the patent to Brighton. This assembly is formed by pressing the covered ceramic trap into a cylindrical housing having a fixed diameter. Again, unacceptable mounting pressures can be experienced should the "stacked" tolerances fall outside of acceptable ranges.
Experience has demonstrated that the manufacture of particulate traps having acceptable service life requires very close manufacturing tolerances of the trap element and surrounding support structure. These tolerances are so sensitive that to hold them consistently adds very significantly to the cost of manufacture of the individual components and may, in fact, be technically unfeasible as a practical matter.
Another problem encountered with the current mounting design lies in the use of end retention rings. These rings are attached to the ends of the trap element to provide axial retention of the trap in cases where "stacked tolerances" result in inadequate mounting pressure so that subsequent thermal expansion differences between the trap element and clam shell results in a loose trap element as discussed above. However, in serving this function, the rings cover approximately the outer 3/8" of the periphery of the trap's inlet and outlet surfaces and therefore reduce the effective filter cross sectional surface area and particulate trapping capacity of the trap near the inlet and outlet of the flow path through the trapping material. As a result, this outer peripheral volume along the length of the trap experiences higher radial temperature gradients and stresses. These stresses become especially severe during engine transients and regeneration and may lead to premature failure of the trapping element.
Outside of the particulate trap mounting art, it has been known to wrap a cylindrical element, such as a catalyst carrier, with a piece of sheet metal to exert a specified pressure on the surface of the catalyst body. The U.S. Pat. No. to Siebels (4,148,120) discloses a catalyst carrier assembly of this type. Specifically, the ends of the rectangular piece of sheet metal are pulled around the catalyst carrier and overlapped. The ends continue to be pulled together until a specified radial pressure is produced on the surface of the catalyst carrier. The ends of the sheet metal are then fastened to maintain the specified radial pressure. The parent patent of Siebels, U.S. Pat. No. 4,093,423, issued to Neumann, also discloses an advantage of this pre-stressed catalytic housing in eliminating many tolerance problems of the carrier diameter in relation to the housing diameter. No suggestion exists to indicate how the catalyst mounting technique could be used in particulate trap mounting.
Therefore, a need exists for a simple, effective and reliable particulate trap mounting system which overcomes the prior art deficiencies noted above.